In semiconductor device fabrication, typically silicon wafers have formed on them thin films such as photoresist ("resist"). The photoresist is conventionally used for defining patterns on the surface of the wafer. For instance, the photoresist is deposited over a layer of e.g. silicon dioxide on the wafer. A pattern is then imaged in the photoresist by photolithography and the photoresist is developed, defining a pattern in the photoresist. An etchant is then applied to the structure, etching away the portions of the silicon dioxide layer which are exposed by the development of the photoresist.
The present inventors have identified the problem that it is often difficult to detect the presence or absence of such photoresist films, especially on a wafer which has already been partially patterned. It is to be understood that the semiconductor fabrication process involves a series of depositions of such photoresist layers and their development and subsequent patterning and etching to build up the semiconductor structure as a stack of layers which are each patterned individually. Since the photoresist films are very thin and often have the same visual characteristics as the underlying portion of the patterned wafer, detecting them by visual observation is often difficult or even impossible. Currently the only sure way to confirm the presence (or absence) of a photoresist film is by actually exposing the photoresist and developing it. Of course, this may be problematic in terms of yield reduction and excess processing steps. Thus this is especially problematic when photoresist is absent. There is no known prior art directed specifically toward such thin film detection, especially in the semiconductor fabrication context.